1. Field of the Invention
This invention concerns methods and apparatuses for analyzing medical imaging data of a subject from an imaging modality using a tracer.
2. Description of the Prior Art
In the medical imaging field, several nuclear medicine emission imaging schemes are known. For example PET (Positron Emission Tomography) is a method for imaging a subject in 3D using an ingested radio-active substance which is processed in the body, typically resulting in an image indicating one or more biological functions. FDG, for instance, is a glucose analog which is used as the radiopharmaceutical tracer in PET imaging to show a map of glucose metabolism. For cancer, for example, FDG is particularly indicated as most tumors are hypermetabolic, which will appear as a high intensity signal in the PET image. For this reason, PET imaging is widely used to detect and stage a wide variety of cancers. The level of glucose activity is usually highly correlated with the aggressiveness and extent of the cancer, and, for example, a reduction in FDG signal between a baseline and a follow-up scan is often indicative of a positive response to therapy.
A key criterion used in evaluating suspicious lesions in a PET scan is the Standardized Uptake Value (SUV). This value is computed from the number of counts of emission events recorded per voxel in the image reconstructed from the event data captured in the PET scan (coincidence emission events along the line of response). The SUV value can also, for example, be adjusted with the intention of accounting for differences in body mass/composition and concentration of radiotracer injected. Effectively the SUV's purpose is to provide a standardized measure of the spatial distribution of radiotracer concentration throughout the imaged portion of the body.
The concentration of radiotracer accumulating in any given tissue region in the body is dependent upon both the affinity of that tissue region for the tracer and the supply of tracer to that tissue region.
Conventionally, PET scans are acquired using a static protocol, producing a single image volume representing the average counts (per voxel) detected over a fixed, short period of time following a given interval between radiotracer injection and image acquisition. This is in contrast to a dynamic protocol, where data is acquired from the time of injection of the tracer, over a much longer period, (e.g. two hours).
It can be difficult in FDG PET imaging to differentiate various levels of glucose metabolism based solely on intensity in a static image. FIG. 1 is an image of a lung cancer patient imaged with FDG PET, where areas of high metabolism are shown in dark. It is expected to find in the body areas of normally high metabolism, for example, the liver (108), heart (104), brain, kidneys (106), and sometimes identification of malignancies (102) in these areas can be very challenging.
Moreover, even if normal high metabolism can be differentiated thanks to a sophisticated knowledge of anatomy and function of each organ, FDG PET can also show high uptake in regions of inflammation. Cellular mediators of inflammation (e.g., mononuclear cells such as monocytes and lymphocytes), along with malignant cells, also have elevated glucose, and therefore FDG, uptake.
Differentiation of malignancy from inflammation is therefore very difficult. It is known that over the first two hours after the injection of FDG, malignant cells will continue to take up FDG whereas inflamed cells will take up FDG and then wash it out progressively (or at least plateau). In FIG. 2, these time-activity curves represent schematically the different uptake patterns over time of FDG in cancer cells (202) and inflamed cells (204). The two dashed lines represent the beginning (206) and end (208) of a putative scanning time (static scan periods are typically between 1 and 10 minutes, depending on the acquisition protocol); the textured pattern in between the two represents the time during which data would be acquired to generate an image.
An image could be acquired so that most of the wash out has occurred, within reasonable time limits. However, in clinical routine, it is not easy to wait for too long a time, because:                a. the decay of the radioactive label forces the image acquisition to be made reasonably soon after the injection so that the image is not too noisy        b. the longer the time the patient has to wait, the longer a room needs to be setup for the patient, which can cause some difficulty in the hospital logistics.        
For these reasons, the image is usually acquired after 30 or 45 minutes, which is not long enough for the wash out to have completely happened. Therefore, the inflammation signal can still be present. Moreover, it may not in any case be possible to differentiate, from a single scan, inflammation from cancer as the relative level of uptake can be similar, depending on the patho-physiological conditions of the patient. FIG. 3 is a schematic illustration of a consequence of such imaging before wash out of tracer from inflammation. Again, the two dashed lines represent the beginning (306) and end (308) of the scanning time and the textured pattern in between represents the time during which data is acquired to generate an image. In this situation, intensity alone (i.e., mean activity measured during acquisition) would not allow differentiation of cancer (302) from inflammation (304), as the two tissues types are in this case at similar activity levels during the acquisition period.
Two protocols have been considered in order to differentiate these tissue types:
1) Dynamic protocol: a scan is acquired from injection of the tracer until long enough for the wash out of inflammation to start: pharmaco-kinetic analysis or clustering techniques can then be applied to differentiate the inflammation from the cancer. However, these scans can take a long time (e.g. two hours) and are not usually acceptable to perform in a clinical routine environment.
2) Dual time point scan (see FIG. 4): two scans are obtained at different time points (406, 408), after, say 60 and 90 minutes. If the uptake goes down between the time points (412), then it is assumed to be inflammation; if it goes up or stay stable (410), it is assumed to be cancer. These kind of protocols are also time consuming and not usually acceptable in a clinical environment.